The new MarketWatch Corporate Tax Calculator shows tax rates for all S&P 500 companies and sectors

The U.S. Senate passed its tax reform bill on Dec. 2, following the House of Representatives, which passed its own version on Nov. 16. There are many differences between the two bills, but they have one major feature in common: both lower the federal corporate income-tax rate to 20% from 35%.

The new MarketWatch Corporate Tax Calculator allows you to look at average income-tax rates for individual S&P 500 companies and for the benchmark index's 11 sectors.

President Trump has said a radical lowering of the federal corporate income-tax rate would make the U.S. more competitive with other countries. Tax inversions -- transactions through which U.S. companies moves theirs domiciles to countries with lower tax rates -- have become a trend in recent years, although the practice began to be curtailed somewhat by the Obama administration in 2014.

Read:Here's how much inversions take from the U.S. Treasury (http://www.marketwatch.com/story/heres-how-much- inversions-take-from-the-us-treasury-2017-09-18)

You might be wondering what the largest companies are paying.

Here are the median effective annualized income-tax rates for the largest five companies that are publicly traded in the U.S., by market value as of Dec. 1:

As you can see, it appears that Facebook Inc.(FB) and Amazon.com Inc.(AMZN) and their shareholders have quite a bit riding on the ultimate form of the tax reform bill that both houses pass and the president signs.

Real estate investment trusts tend not to pay income taxes, or to pay very little, because most of their income is passed through to their shareholders. If we leave the REIT sector aside, here are the five S&P 500 companies with the lowest median effective annualized income-tax rates:

The new MarketWatch Corporate Tax Calculator is below. It includes average effective annual income-tax rates for the entire S&P 500 index . If you click the search box, you can select one of the 11 sectors of the index, to see its own average effective tax rate, as well as those for the five companies in that sector with the highest average rates and the five with the lowest. You can also search alphabetically for any S&P 500 company.

Companies' effective income-tax rates can change radically from quarter to quarter and year to year because of major accounting adjustments and events. Some examples are asset write-downs, gains or losses on sales, realization of deferred tax assets and legal settlements. So we used data provided by FactSet to calculate median effective annual income-tax rates based on the past 11 years' financial statements from companies annual 10-K filings with the Securities and Exchange Commission. If less than five years of data were available for a company, we did not calculate its median tax rate.

For the sector data, FactSet took a further step to smooth out the results, by excluding any companies annual income- tax rates that were above 100% or below 0%.

Another point about the data: the effective income-tax rates include state and local income taxes paid, if any.

If a company you are interested in has a high median effective annual income-tax rate, it's obvious that quite a bit is riding on whether or not the two houses of Congress pass a joint bill that President Trump is willing to sign. But keep in mind that many differences between the two tax reform versions need to be worked out.

Much more on tax reform: The Trump tax calculator -- will you pay more or less? (http://www.marketwatch.com/story/the- new-trump-tax-calculator-what-do-you-owe-2017-10-26)